Liquid crystal display device

ABSTRACT

A liquid crystal display device with a novel structure is provided. Each pixel includes a first circuit for holding a high level (or low level) potential and a second circuit for holding a low level (or high level) potential. A semiconductor layer of a transistor included in each of the first and second circuits is an oxide semiconductor layer. The second circuit is reset when being supplied with the high level potential. Whether the high level potential held in the second circuit changes is controlled by a data voltage supplied to the first circuit. The potential held in the first circuit and the potential held in the second circuit are respectively supplied to a first transistor and a second transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a display device, inparticular, a liquid crystal display device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of the invention disclosed inthis specification and the like relates to an object, a method, or amanufacturing method. Furthermore, the present invention relates to aprocess, a machine, manufacture, or a composition of matter.Specifically, examples of the technical field of one embodiment of thepresent invention disclosed in this specification include asemiconductor device, a display device, a light-emitting device, a powerstorage device, a storage device, a method for driving any of them, anda method for manufacturing any of them.

2. Description of the Related Art

To reduce power consumption, a reflective liquid crystal display deviceprovided with a memory in each pixel has been proposed (see PatentDocument 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2005-236814

SUMMARY OF THE INVENTION

In the case where a memory equivalent to a static random access memory(SRAM) is provided in a pixel as disclosed in Patent Document 1,miniaturization of transistors constituting the SRAM is required aspixel resolution increases. This leads to a higher leakage current ofthe transistors, which causes difficulty in reducing power consumption.

In view of the above, an object of one embodiment of the presentinvention is to provide a low-power liquid crystal display device or thelike having a novel structure. Another object of one embodiment of thepresent invention is to provide a liquid crystal display device or thelike having a novel structure which allows a reduction in the off-statecurrent flowing through a transistor in a pixel. Another object of oneembodiment of the present invention is to provide a novel liquid crystaldisplay device or the like.

Note that the objects of the present invention are not limited to theabove objects. The above objects do not disturb the existence of otherobjects. The other objects are objects that are not described above andwill be described below. The other objects will be apparent from and canbe derived as appropriate from the description of the specification, thedrawings, and the like by those skilled in the art. One embodiment ofthe present invention achieves at least one of the above objects and/orthe other objects.

One embodiment of the present invention is a liquid crystal displaydevice including a pixel provided with a first circuit capable ofholding one of a first potential and a second potential of a datavoltage, a second circuit capable of holding the other potential of thedata voltage, a first transistor, and a second transistor. The onepotential of the data voltage is supplied to a gate of the firsttransistor, and the other potential of the data voltage is supplied to agate of the second transistor. One of a source and a drain of each ofthe first and second transistors is electrically connected to aconductive layer serving as a pixel electrode, the other of the sourceand the drain of the first transistor is electrically connected to awiring supplied with a signal allowing light to pass through a liquidcrystal layer, and the other of the source and the drain of the secondtransistor is electrically connected to a wiring supplied with a signalpreventing light from passing through the liquid crystal layer. Asemiconductor layer of a transistor included in each of the first andsecond circuits is an oxide semiconductor layer.

One embodiment of the present invention is a liquid crystal displaydevice including a pixel provided with a first circuit capable ofholding one of a first potential and a second potential of a datavoltage, a second circuit capable of holding the other potential of thedata voltage, a first transistor, and a second transistor. The onepotential of the data voltage is supplied to a gate of the firsttransistor, and the other potential of the data voltage is supplied to agate of the second transistor. One of a source and a drain of each ofthe first and second transistors is electrically connected to aconductive layer serving as a pixel electrode, the other of the sourceand the drain of the first transistor is electrically connected to awiring supplied with a signal allowing light to pass through a liquidcrystal layer, and the other of the source and the drain of the secondtransistor is electrically connected to a wiring supplied with a signalpreventing light from passing through the liquid crystal layer. Thefirst potential is higher than the second potential. The first circuitis controlled so that the second potential is held when a scan signal issupplied and the first or second potential is held when the data voltageis then applied to the first circuit. The second circuit is controlledso that the first potential is held when a reset signal is supplied anda potential different from that held in the first circuit is held whenthe data voltage is then applied to the first circuit. A semiconductorlayer of a transistor included in each of the first and second circuitsis an oxide semiconductor layer.

According to one embodiment of the present invention, it is possible toprovide a low-power liquid crystal display device or the like having anovel structure. Alternatively, according to one embodiment of thepresent invention, it is possible to provide a liquid crystal displaydevice or the like having a novel structure which allows a reduction inthe off-state current flowing through a transistor in a pixel.Alternatively, according to one embodiment of the present invention, itis possible to provide a novel liquid crystal display device or thelike.

Note that the effects of the present invention are not limited to theabove effects. The above effects do not disturb the existence of othereffects. The other effects are effects that are not described above andwill be described below. The other effects will be apparent from and canbe derived as appropriate from the description of the specification, thedrawings, and the like by those skilled in the art. One embodiment ofthe present invention has at least one of the above effects and/or theother effects. Accordingly, one embodiment of the present invention doesnot have the above effects in some cases.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a circuit diagram illustrating one embodiment of the presentinvention, and FIG. 1B is a timing chart thereof;

FIGS. 2A and 2B are circuit diagrams illustrating one embodiment of thepresent invention;

FIGS. 3A and 3B are circuit diagrams illustrating one embodiment of thepresent invention;

FIGS. 4A and 4B are schematic views of waveforms illustrating oneembodiment of the present invention;

FIG. 5 is a block diagram illustrating one embodiment of the presentinvention;

FIGS. 6A to 6C are block diagrams each illustrating one embodiment ofthe present invention;

FIG. 7 is a circuit diagram illustrating one embodiment of the presentinvention;

FIG. 8 is a circuit diagram illustrating one embodiment of the presentinvention;

FIGS. 9A and 9B are cross-sectional schematic views each illustratingone embodiment of the present invention;

FIGS. 10A to 10C are cross-sectional schematic views each illustratingone embodiment of the present invention;

FIGS. 11A and 11B are conceptual diagrams illustrating examples of adriving method of a liquid crystal display device;

FIG. 12 illustrates a display module;

FIGS. 13A to 13H are external views of electronic devices of oneembodiment;

FIGS. 14A to 14H are external views of electronic devices of oneembodiment;

FIGS. 15A to 15C are cross-sectional TEM images and a local Fouriertransform image of an oxide semiconductor;

FIGS. 16A and 16B show nanobeam electron diffraction patterns of oxidesemiconductor films, and FIGS. 16C and 16D illustrate an example of atransmission electron diffraction measurement apparatus;

FIG. 17A shows an example of structural analysis by transmissionelectron diffraction measurement, and FIGS. 17B and 17C show planar TEMimages;

FIG. 18 is a circuit diagram illustrating one embodiment of the presentinvention;

FIG. 19 is a circuit diagram illustrating one embodiment of the presentinvention;

FIG. 20 is a circuit diagram illustrating one embodiment of the presentinvention;

FIG. 21 is a circuit diagram illustrating one embodiment of the presentinvention;

FIGS. 22A and 22B are circuit diagrams illustrating one embodiment ofthe present invention; and

FIG. 23 is a circuit diagram illustrating one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below with reference to the drawings. Notethat the embodiments can be implemented in various different ways and itwill be readily appreciated by those skilled in the art that modes anddetails of the present invention can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Thepresent invention therefore should not be construed as being limited tothe following description of the embodiments. Note that in structures ofthe invention described below, reference numerals denoting the sameportions are used in common in different drawings.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Thus, embodiments of the presentinvention are not limited to such scales. Note that the drawings areschematic views showing ideal examples, and embodiments of the presentinvention are not limited to shapes or values shown in the drawings. Forexample, the following can be included: variation in signal, voltage, orcurrent due to noise or difference in timing.

In this specification and the like, a transistor is an element having atleast three terminals: a gate, a drain, and a source. The transistorincludes a channel region between the drain (a drain terminal, a drainregion, or a drain electrode) and the source (a source terminal, asource region, or a source electrode) and current can flow through thedrain, the channel region, and the source.

Here, since the source and the drain of the transistor change dependingon the structure, the operating condition, and the like of thetransistor, it is difficult to define which is a source or a drain.Thus, a region that functions as a source or a region that functions asa drain is not referred to as a source or a drain in some cases. In thatcase, one of the source and the drain might be referred to as a firstterminal, and the other of the source and the drain might be referred toas a second terminal.

In this specification, ordinal numbers such as “first,” “second,” and“third” are used to avoid confusion among components, and thus do notlimit the number of the components.

In this specification, the expression “A and B are connected” means thecase where “A and B are electrically connected” in addition to the casewhere “A and B are directly connected.” Here, the expression “A and Bare electrically connected” means the case where electric signals can betransmitted and received between A and B when an object having anyelectric action exists between A and B.

In this specification, terms for describing arrangement, such as “over”and “under,” are used for convenience for describing the positionalrelationship between components with reference to drawings. Furthermore,the positional relationship between components is changed as appropriatein accordance with a direction in which each component is described.Thus, there is no limitation on terms used in this specification, anddescription can be made appropriately depending on the situation.

The positional relationships of circuit blocks in diagrams are specifiedfor description, and even in the case where different circuit blockshave different functions in the diagrams, the different circuit blocksmight be provided in an actual circuit or region so that differentfunctions are achieved in the same circuit block. The functions ofcircuit blocks in diagrams are specified for description, and even inthe case where one circuit block is illustrated, blocks might beprovided in an actual circuit or region so that processing performed byone circuit block is performed by a plurality of circuit blocks.

Voltage refers to a difference between a given potential and a referencepotential (e.g., a ground potential) in many cases. Thus, voltage, apotential, and a potential difference can also be referred to as apotential, voltage, and a voltage difference, respectively. Note thatvoltage refers to a difference between potentials of two points, and apotential refers to electrostatic energy (electric potential energy) ofa unit charge at a given point in an electrostatic field.

Note that in general, a potential and voltage are relative values. Thus,a ground potential is not always 0 V.

In this specification and the like, the term “parallel” indicates thatan angle formed between two straight lines is −10° to 10°, andaccordingly includes the case where the angle is −5° to 5°. In addition,the term “perpendicular” indicates that an angle formed between twostraight lines is 80° to 100°, and accordingly includes the case wherethe angle is 85° to 95°.

In this specification and the like, trigonal and rhombohedral crystalsystems are included in a hexagonal crystal system.

EMBODIMENT 1

In this embodiment, a liquid crystal display device that is oneembodiment of the present invention will be described with reference todrawings.

Note that the liquid crystal display device might also be referred to asa display module including a display controller, a power supply circuit,a backlight unit, and the like provided over a separate substrate.

FIG. 1A is a circuit diagram of a pixel 10 included in the liquidcrystal display device. FIG. 1B is a timing chart showing the operationof the pixel 10.

The pixel 10 illustrated in FIG. 1A includes a first circuit 11, asecond circuit 13, a transistor 15, a transistor 17, and a liquidcrystal element 14.

The first circuit 11 includes a transistor 23 and a capacitor 25.

The second circuit 13 includes a transistor 27, a transistor 29, and acapacitor 31.

The transistor 15 is turned on or off when a potential held in the firstcircuit 11 is supplied to its gate. The transistor 17 is turned on oroff when a potential held in the second circuit 13 is supplied to itsgate. Note that the transistor 15 and the transistor 17 are alsoreferred to as a first transistor and a second transistor, respectively,in some cases.

One of a source and a drain of the transistor 15, and one of a sourceand a drain of the transistor 17 are connected to one electrode of theliquid crystal element 14. Note that the one electrode of the liquidcrystal element 14 is a conductive layer serving as a pixel electrode.The other electrode of the liquid crystal element 14 has a function as acommon electrode to which a common voltage (Vcom) is applied.

The other of the source and the drain of the transistor 15 iselectrically connected to a wiring 19 supplied with a signal that allowslight to pass through a liquid crystal layer. Note that when thetransistor 15 is on, a voltage V1 applied to the wiring 19 controls thealignment of the liquid crystal layer between the electrodes of theliquid crystal element 14 so that light is transmitted. The electricfield generated between the common voltage Vcom and the voltage V1 maybe either a positive electric field or a negative electric field. Forexample, a signal varying around the common voltage may be supplied asthe voltage V1 so that the positive electric field and the negativeelectric field are alternately applied.

The other of the source and the drain of the transistor 17 iselectrically connected to a wiring 21 supplied with a signal thatprevents light from passing through the liquid crystal layer. Note thatwhen the transistor 17 is on, a voltage V2 applied to the wiring 21controls the alignment of the liquid crystal layer between theelectrodes of the liquid crystal element 14 so that light is nottransmitted. The common voltage Vcom is equal to the voltage V2. Whenthe common voltage Vcom is a constant voltage, a signal supplied to thewiring 21 also has a constant voltage. When the common voltage Vcom is avarying signal, the signal supplied to the wiring 21 varies similarly.

The one of the pair of electrodes of the liquid crystal element 14 has apotential based on a data signal. The other of the pair of electrodes ofthe liquid crystal element 14 has a common potential (Vcom).

The liquid crystal element 14 is an element that has a function ofcontrolling transmission or non-transmission of light utilizing anoptical modulation action of liquid crystal. Note that the opticalmodulation action of the liquid crystal is controlled by an electricfield applied to the liquid crystal (including a horizontal electricfield, a vertical electric field, and a diagonal electric field).Examples of the liquid crystal element 14 are a nematic liquid crystal,a cholesteric liquid crystal, a smectic liquid crystal, a thermotropicliquid crystal, a lyotropic liquid crystal, a ferroelectric liquidcrystal, and an anti-ferroelectric liquid crystal.

The display device including the liquid crystal element 14 can be drivenby any of the following modes: a TN mode, a VA mode, an ASM (axiallysymmetric aligned micro-cell) mode, an OCB (optically compensatedbirefringence) mode, an MVA mode, a PVA (patterned vertical alignment)mode, an IPS mode, an FFS mode, a TBA (transverse bend alignment) mode,and the like. Note that one embodiment of the present invention is notlimited to the above, and various liquid crystal elements and drivingmethods can be used.

The liquid crystal element may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less and is optically isotropic; therefore,alignment treatment is not necessary and viewing angle dependence issmall.

Note that in one embodiment of the present invention, the electrodeserving as the pixel electrode of the liquid crystal element 14preferably has a function of reflecting external light. In other words,the liquid crystal display device is preferably a reflective liquidcrystal display device. The use of external light for making displayedimages visible reduces the power consumed by a backlight, resulting inlower power consumption. Note that the electrode serving as the pixelelectrode of the liquid crystal element 14 may be a light-transmittingelectrode, or a combination of a light-transmitting electrode and anelectrode reflecting external light.

The first circuit 11 is controlled so that a second potential is heldwhen a scan signal is supplied and either a first potential or thesecond potential is held when a data voltage Vsig based on the datasignal is then applied to the first circuit 11.

The second circuit 13 is controlled so that the first potential is heldwhen a reset signal is supplied and a potential that is different fromthe potential held in the first circuit 11 is held when the data voltageVsig is then applied to the first circuit 11.

Note that the first potential is based on a high power supply potentialVdd, whereas the second potential is based on a low power supplypotential Vss. In other words, the data signal is a two-level signal andthe data voltage Vsig is a two-level voltage. Note that the firstpotential is higher than the second potential.

Note that the first potential is preferably higher than a potentialsupplied as the signal that allows light to pass through the liquidcrystal layer. The second potential is preferably lower than thepotential supplied as the signal that allows light to pass through theliquid crystal layer. With such a structure, the transistors 15 and 17can be turned on or off more surely.

The transistor 23 controls writing of a data voltage to the firstcircuit 11 and holding of the data voltage therein. A gate of thetransistor 23 is connected to a wiring 33 to which a voltage Vg based ona scan signal is applied. One of a source and a drain of the transistor23 is connected to a wiring 35 to which the data voltage Vsig based on adata signal is applied. The other of the source and the drain of thetransistor 23 is connected to the gate of the transistor 15, oneelectrode of the capacitor 25, and a gate of the transistor 27. Thetransistor 23 is also referred to as a third transistor.

Note that in the following description, a node at which the other of thesource and the drain of the transistor 23, the gate of the transistor15, the one electrode of the capacitor 25, and the gate of thetransistor 27 are connected one another is referred to as a node A(NodeA). Note that the node A allows one of the first and secondpotentials written by the application of the data voltage Vsig to beheld in the first circuit 11.

The capacitor 25 allows one of the first and second potentials writtenby the application of the data voltage Vsig to be held in the firstcircuit 11. The capacitor 25 can be omitted in the case where thetransistor 15 or 27 has a large gate capacitance or the node A has alarge parasitic capacitance; an example of that case is illustrated inFIG. 20. Note that the capacitor 25 is also referred to as a firstcapacitor. Although the other electrode of the capacitor 25 is connectedto a wiring 37 supplied with the second potential in FIG. 1A, the otherelectrode of the capacitor 25 may be connected to any wiring suppliedwith a fixed potential, e.g., a wiring 41 supplied with the firstpotential, or the wiring 21 as illustrated in FIG. 18.

By the control of the transistor 27, the first or second potential thatis different from the potential held in the first circuit 11 is held inthe second circuit 13. The gate of the transistor 27 is connected to theother of the source and the drain of the transistor 23, the gate of thetransistor 15, and the one electrode of the capacitor 25, i.e., the nodeA. One of a source and a drain of the transistor 27 is connected to thewiring 37 supplied with the second potential. The other of the sourceand the drain of the transistor 27 is connected to the gate of thetransistor 17, one electrode of the capacitor 31, and the other of asource and a drain of the transistor 29. The transistor 27 is alsoreferred to as a fifth transistor.

Note that in the following description, a node at which the other of thesource and the drain of the transistor 27, the gate of the transistor17, the one electrode of the capacitor 31, and the other of the sourceand the drain of the transistor 29 are connected one another is referredto as a node B (NodeB). Note that the node B allows the other of thefirst and second potentials written by the application of the datavoltage Vsig to be held in the second circuit 13.

The capacitor 31 allows the other of the first and second potentialswritten by the application of the data voltage Vsig to be held in thesecond circuit 13. The capacitor 31 can be omitted in the case where thetransistor 17 has a large gate capacitance or the node B has a largeparasitic capacitance; an example of that case is illustrated in FIG.20. Note that the capacitor 31 is also referred to as a secondcapacitor. Although the other electrode of the capacitor 31 is connectedto the wiring 37 supplied with the second potential, the other electrodeof the capacitor 31 may be connected to a wiring supplied with a fixedpotential, e.g., the wiring 41 supplied with the first potential. Notethat different potentials may be supplied to the other electrode of thecapacitor 31 and the other electrode of the capacitor 25; an example ofthat case is illustrated in FIG. 19.

By the control of the transistor 29, the node B is reset (initialized)to the first potential in advance so that the first or second potentialthat is different from the potential held in the first circuit 11 isheld in the second circuit 13. A gate of the transistor 29 is connectedto a wiring 39 supplied with a voltage Vres based on a reset signal ofthe transistor 29. One of the source and the drain of the transistor 29is connected to the wiring 41 supplied with the first potential. Theother of the source and the drain of the transistor 29 is connected tothe gate of the transistor 17, the one electrode of the capacitor 31,and the other of the source and the drain of the transistor 27. Thetransistor 29 is also referred to as a fourth transistor.

In order that charge keeps being held at the node A and the node B, eachof the transistors 15, 17, 23, 27, and 29 in the pixel 10 is preferablya transistor with a low off-state current (a low current flowing betweena source and a drain in a non-conductive state). Accordingly, asemiconductor layer that includes a channel formation region in each ofthe transistors 15, 17, 23, 27, and 29 is preferably, for example, asemiconductor layer including an oxide semiconductor. Note that oneembodiment of the present invention is not limited to this example. Thesemiconductor layer may include amorphous silicon, polycrystallinesilicon, single crystal silicon, or the like.

Note that the term “low off-state current” means that a normalizedoff-state current per micrometer of a channel width at room temperaturewith a source-drain voltage of 10 V is less than or equal to 10 zA. Theoxide semiconductor will be described later in detail.

Note that FIG. 1A shows an example in which only N-channel transistorsare used; however, one embodiment of the present invention is notlimited to this example. FIG. 23 shows an example in which onlyP-channel transistors are used.

With the aforementioned pixel structure in the liquid crystal displaydevice that is one embodiment of the present invention, a potentialcorresponding to data can be held in the pixel while charge is consumedas little as possible unlike in an SRAM. Furthermore, with the pixelstructure of the liquid crystal display device that is one embodiment ofthe present invention, a liquid crystal element is not directlyconnected to a node where charge is held. This reduces the leakage ofcharge through the liquid crystal element, preventing data loss andreducing power consumption.

Also with the pixel structure in the liquid crystal display device thatis one embodiment of the present invention, a written data voltage canbe held during the period almost equal to that in an SRAM. Hence, theoperation of a driver circuit can be stopped when a still image isdisplayed, whereby the power consumption can be reduced.

Also with the pixel structure in the liquid crystal display device thatis one embodiment of the present invention, the power consumed by abacklight can be reduced because a reflective liquid crystal displaydevice is used. In the reflective liquid crystal display device,transistors in a pixel can be provided to overlap with a pixelelectrode. Therefore, even when the number of transistors in the pixelincreases, the pixel size is not reduced and display quality can bemaintained.

Next, the operation of the pixel illustrated in FIG. 1A will bedescribed with reference to a timing chart in FIG. 1B. The timing chartin FIG. 1B shows the operation in a reset period (initializationperiod), a blank period, and a data voltage writing period. In addition,possible conductive states of the transistors in each period areillustrated in FIGS. 2A and 2B and FIGS. 3A and 3B, and described withthe timing chart. Note that each signal is a two-level signal; thus, inthe drawings, a high level signal and a low level signal are abbreviatedto as H and L, respectively. In the following description, it is assumedthat the first potential and the second potential of the data voltageVsig are at the high level and the low level, respectively.

FIGS. 2A and 2B and FIGS. 3A and 3B also illustrate changes in thepotential at the aforementioned node A (NodeA) and node B (NodeB), andalso illustrate a change in the potential of the one electrode of theliquid crystal element 14 (NodeLC). Furthermore, in FIGS. 2A and 2B andFIGS. 3A and 3B, the transistors 15, 17, 23, 27, and 29 are marked by Xwhen being off.

First, in the reset period (Tres in FIG. 1B and FIG. 2A), the voltage Vgbased on a scan signal is at the high level (denoted by H in thedrawings), the voltage Vres based on a reset signal is at the highlevel, and the data voltage based on a data signal is at the low level(denoted by L in the drawings). Thus, the transistors 23 and 29 areturned on, so that the node A and the node B are at the low level andthe high level, respectively. Moreover, the node NodeLC has the voltageV2 because the transistor 17 is turned on.

The blank period is not necessarily provided if the operation isperformed smoothly.

In the blank period (Tb in FIG. 1B and FIG. 2B), the voltage Vg based ona scan signal is at the high level, the voltage Vres based on a resetsignal is at the low level, and the data voltage based on a data signalis at the low level. Thus, the transistor 23 is on, so that the node Ais kept at the low level and the node B is kept at the high level.Moreover, the node NodeLC has the voltage V2 because the transistor 17is kept on.

Then, the data voltage writing period is described. First, descriptionis made on the case where the data voltage Vsig based on a data signalis at the high level. In the data voltage writing period at the timewhen the data voltage Vsig is at the high level (Tsig(H) in FIG. 1B andFIG. 3A), the voltage Vg based on a scan signal is at the high level,the voltage Vres based on a reset signal is at the low level, and thedata voltage based on a data signal is at the high level. Thus, thetransistors 23 and 27 are turned on, so that the node A and the node Bare at the high level and the low level, respectively. Moreover, thenode NodeLC has the voltage V1 because the transistor 15 is turned on.

Next, description is made on the data voltage writing period at the timewhen the data voltage Vsig based on a data signal is at the low level.In the data voltage writing period at the time when the data voltageVsig is at the low level (Tsig(L) in FIG. 1B and FIG. 3B), the voltageVg based on a scan signal is at the high level, the voltage

Vres based on a reset signal is at the low level, and the data voltagebased on a data signal is at the low level. Thus, the transistor 23 ison, so that the node A is at the low level and the node B is at the highlevel. Moreover, the node NodeLC has the voltage V2 because thetransistor 17 is turned on.

Note that the data voltage that has been written in the data voltagewriting period can be maintained until another reset signal or scansignal is supplied. To achieve this, for example, in the case where thenode A is kept at the high level, the voltage based on a scan signal isset at the low level so that the transistor 23 is turned off. In thatcase, the transistor 27 is on and the node B can be kept at the lowlevel, which is opposite to the level of the node A.

In the case where the node A is kept at the low level, the voltage baseda scan signal is set at the low level so that the transistor 23 isturned off. In that case, the transistors 27 and 29 are also off and thenode B can be kept at the high level, which is opposite to the level ofthe node A.

In a period during which the data voltage written in the data voltagewriting period is held, charge held in the node A and the node B hardlymoves when the transistors 23, 27, and 29 each have a low off-statecurrent. Accordingly, when the conductive states of the transistors 15and 17 are kept, an image continues to be displayed even when a datasignal is not written. In that case, a driver circuit for supplying adata signal to a pixel, a driver circuit for supplying a scan signal tothe pixel, and a driver circuit for supplying a reset signal to thepixel can be stopped operating, reducing power consumption.

In the aforementioned operation of the pixel in the liquid crystaldisplay device that is one embodiment of the present invention, apotential corresponding to data can be held in the pixel as data withdifferent values of two nodes. Also in the operation of the pixel of oneembodiment of the present invention, data can be stored in a mannersimilar to that in an SRAM. In addition, an increase in powerconsumption with miniaturization, which is caused in an SRAM, can beprevented, and charge therefore can be consumed as little as possible.

Note that one embodiment of the present invention is not limited to thecircuit structure illustrated in FIG. 1A. Circuit structures other thanthe structure in FIG. 1A may be employed when, for example, theconnection structure is as shown in FIG. 21 in the reset period and isas shown in FIG. 22A or FIG. 22B in the data voltage writing period.

Also in the operation of the pixel in the liquid crystal display devicethat is one embodiment of the present invention, a written data voltagecan be held in the first circuit and the second circuit equivalent to anSRAM. Hence, the operation of a driver circuit can be stopped when astill image is displayed, whereby the power consumption can be reduced.

FIGS. 4A and 4B are schematic views of signal waveforms showing examplesof the voltage V1 based on a signal allowing light to pass through aliquid crystal layer and the voltage V2 based on a signal preventinglight from passing through the liquid crystal layer.

As shown in FIG. 4A, the voltage V1 based on the signal allowing lightto pass through the liquid crystal layer may be generated so as to varybetween a potential VH and a potential VL with a potential Vctherebetween every frame. Such a structure reduces degradation of liquidcrystals contained in the liquid crystal layer. The common voltage Vcomand the voltage V2 based on the signal preventing light from passingthrough the liquid crystal layer may be generated so as to be equal toeach other, here, they have the potential Vc.

The highest potential VH of the voltage V1 is preferably lower than thefirst potential supplied to the node A and the node B, namely, thepotential Vdd. Furthermore, in one embodiment of the present invention,the lowest potential VL of the voltage V1 is preferably higher than thepotential Vss, which is the second potential supplied to the node A andthe node B. With such a structure, the transistors 15 and 17 can beturned on or off more surely.

Note that the voltage V1, the voltage V2, and the common voltage Vcommay each vary as shown in FIG. 4B. In that case, the signal amplitudecan be lower than that in the case of FIG. 4A even when the same voltageis applied to liquid crystals, reducing power consumption.

FIG. 5 is a block diagram of a liquid crystal display device includingthe aforementioned pixel 10.

The display device illustrated in FIG. 5 includes a pixel portion 43, ascan line driver circuit 45, a reset signal line driver circuit 57, asignal line driver circuit 47 provided with a sampling circuit 51 and ashift register 49, n (n is a natural number) scan lines G1 to Gn whichare parallel or substantially parallel to each other and whosepotentials are controlled by the scan line driver circuit 45, n resetsignal lines R1 to Rn which are parallel or substantially parallel toeach other and whose potentials are controlled by the reset signal linedriver circuit 57, and m (m is a natural number) signal lines S1 to Smwhich are parallel or substantially parallel to each other and whosepotentials are controlled by the signal line driver circuit 47. Thepixel portion 43 includes a plurality of pixels 10 arranged in a matrix.The pixels are connected to a wiring VL1 and a wiring VL2 which supply avoltage V1 and V2, respectively, output from a voltage generator circuit55.

The sampling circuit 51 is supplied with sampling signals Samp_1 toSamp_m output from the shift register 49, and a data signal Vdata outputfrom a digital data generator circuit 53. A data voltage Vsig based onthe data signal Vdata is sampled in the signal lines S1 to Sm andwritten to each pixel when the scan signal is selected. Note that thedata signal Vdata is a two-level voltage signal obtained when image data(data) is converted in the digital data generator circuit 53.

Although the data signal Vdata is a two-level voltage signal in thefollowing description, a three or more level image can also be displayedby combining another grayscale method. For example, an area grayscalemethod may be used in combination to display a multi-level image. Inthat case, each of the pixels 10 serves as a subpixel.

In the case where a multi-level image is displayed by the area grayscalemethod, subpixels are disposed, for example, as shown in FIGS. 6A to 6C.In the case where, for example, pixels performing color display are usedin combination with the area grayscale method, subpixels 44_0 and 44_1with different areas are provided for each color, red, green, and blue(RGB). Although the subpixels with different areas are used in FIG. 6A,all the subpixels may have the same area as illustrated in FIG. 6B andgrayscale display may be performed by weighting the area where lightpasses. Instead of the structures illustrated in FIGS. 6A and 6B, whichinclude the RGB subpixels, a structure including white (W) subpixels inaddition to RGB subpixels can be employed to perform grayscale displayas shown in FIG. 6C.

In the above description, the liquid crystal element is described as anexample of the display element; a different display element can also beused in the pixel of the display device in one embodiment of the presentinvention.

FIG. 7 illustrates a pixel 10EL as another example. The pixel 10EL isprovided in a display device that includes a light-emitting element 59instead of the liquid crystal element 14 in the pixel 10 illustrated inFIG. 1A.

One of a pair of electrodes of the light-emitting element 59 isconnected to one of the source and the drain of each of the transistors15 and 17. The other electrode of the light-emitting element 59 isconnected to a wiring Vcat serving as a cathode. Examples of thelight-emitting element 59 include an organic electroluminescent element(also referred to as an organic EL element). Note that thelight-emitting element 59 is not limited to the organic EL element andmay be an inorganic EL element containing an inorganic material.

Note that although FIG. 7 illustrates an example in which thelight-emitting element 59 is used as a display element, one embodimentof the present invention is not limited thereto. Any of a variety ofdisplay elements can be used. Examples of display elements includeelements including a display medium whose contrast, luminance,reflectance, transmittance, or the like is changed by electromagneticaction, such as an electroluminescent (EL) element (e.g., an EL elementincluding organic and inorganic materials, an organic EL element, or aninorganic EL element), an LED (e.g., a white LED, a red LED, a greenLED, or a blue LED), a transistor (a transistor that emits lightdepending on current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), or a display element using a micro electromechanical system (MEMS), a digital micromirror device (DMD), a digitalmicro shutter (DMS), an interferometric modulator (IMOD) element, a MEMSshutter display element, an optical-interference-type MEMS displayelement, an electrowetting element, a piezoelectric ceramic display, anda carbon nanotube. Examples of display devices including EL elementsinclude an EL display. Examples of display devices including electronemitters are a field emission display (FED) and an SED-type flat paneldisplay (SED: surface-conduction electron-emitter display). Examples ofdisplay devices including liquid crystal elements include a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of a display device using electronic ink orelectrophoretic elements include electronic paper. In a transflectiveliquid crystal display or a reflective liquid crystal display, some ofor all of pixel electrodes function as reflective electrodes. Forexample, some or all of pixel electrodes are formed to contain aluminum,silver, or the like. In such a case, a memory circuit such as SRAM canbe provided under the reflective electrodes, leading to lower powerconsumption.

In the above description, the pixel is used for displaying a two-levelimage; however, in the case where a multi-level image is displayed, thepixel may be switched to another pixel to perform display. For example,as shown in FIG. 8, a pixel 10B is provided adjacent to a pixel 10Awhich corresponds to the pixel 10 illustrated in FIG. 1A. The connectionbetween the liquid crystal element 14 and the pixel 10A or 10B ischanged by a switch 69.

The pixel 10B includes a transistor 61 and a capacitor 63, therebyhaving a so-called one-transistor and one-capacitor structure. Thetransistor 61 is used as a switch. The transistor 61 is turned on or offdepending on a scan signal VgB which is supplied to a wiring 65connected to a gate of the transistor 61. One of a source and a drain ofthe transistor 61 is connected to a wiring 67 supplied with a datavoltage Vsig (Ana.) with an analog value. When the switch 69 is switchedand the data voltage Vsig (Ana.) is written to the capacitor 63connected to the other of the source and the drain of the transistor 61,multi-level display can be performed.

In the aforementioned liquid crystal display device that is oneembodiment of the present invention, a potential corresponding to datacan be held in a pixel without being influenced by charge leakage unlikein an SRAM. With the pixel structure of one embodiment of the presentinvention, a node allowing charge to be held is not directly connectedto a liquid crystal element but is connected to a transistor including asemiconductor layer using an oxide semiconductor. This reduces theleakage of charge through the liquid crystal element, preventing dataloss and reducing power consumption.

Also in the liquid crystal display device that is one embodiment of thepresent invention, a written data voltage can be held in a circuit wheredata can be held during the period almost equal to that in an SRAM.Hence, the operation of a driver circuit can be stopped when a stillimage is displayed, whereby the power consumption can be reduced.

In addition, the liquid crystal display device that is one embodiment ofthe present invention is a reflective liquid crystal display device;accordingly, the power consumed by a backlight can be reduced. In thereflective liquid crystal display device, transistors in a pixel can beprovided to overlap with a pixel electrode. Therefore, even when thenumber of transistors in the pixel increases, the pixel size is notreduced and display quality can be maintained.

The structures shown in this embodiment can be used in appropriatecombination with any of the structures shown in the other embodiments.

EMBODIMENT 2

Described in this embodiment are examples of schematic cross-sectionalstructures that can be used for the liquid crystal display devicedescribed above. Here, structure examples of the schematic cross sectionof the liquid crystal display device will be described with reference tocross-sectional views of FIGS. 9A and 9B. FIGS. 9A and 9B show examplesof the cross-sectional views of a reflective liquid crystal displaydevice.

The cross-sectional schematic view of FIG. 9A illustrates an elementsubstrate 71, a transistor 73, a conductive layer 75 serving as a pixelelectrode, a depression and projection portion 77, a liquid crystal 79,a counter substrate 81, a coloring layer 83, a light-shielding layer 85,an insulating layer 87, a conductive layer 89 serving as a counterelectrode, and a polarizing plate 91.

FIG. 9B illustrates a structure different from that in thecross-sectional schematic view of FIG. 9A. In FIG. 9B, a light diffusionlayer 93 is provided between the counter substrate 81 and the polarizingplate 91 in addition to the components illustrated in FIG. 9A. Such astructure is preferable because the depression and projection portion77, which is formed by processing a surface of a conductive layer, canbe omitted.

The components illustrated in FIGS. 9A and 9B will be described below.

There is no particular limitation on a material and the like of theelement substrate 71 and the counter substrate 81 as long as thematerial has heat resistance high enough to withstand at least heattreatment performed later. For example, a glass substrate, a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be used asthe element substrate 71 and the counter substrate 81. In the case wherea glass substrate is used as the element substrate 71 and the countersubstrate 81, a glass substrate having any of the following sizes can beused: the 6th generation (1500 mm×1850 mm), the 7th generation (1870mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation(2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm) Thus, alarge-sized liquid crystal display device can be manufactured.

Alternatively, a flexible substrate may be used as the element substrate71, and the transistor may be provided directly on the flexiblesubstrate. A separation layer may be provided between the elementsubstrate 71 and the transistor. The separation layer can be used whenpart or the whole of an element portion formed over the separation layeris completed, separated from the element substrate 71, and transferredto another substrate. In such a case, the transistor can be transferredto a substrate having low heat resistance or a flexible substrate aswell.

That is, the transistor can be formed using a variety of substrates.There is no particular limitation on the type of a substrate. Examplesof the substrate include a semiconductor substrate (e.g., a singlecrystal substrate or a silicon substrate), an SOI substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a metal substrate, astainless steel substrate, a substrate including stainless steel foil, atungsten substrate, a substrate including tungsten foil, a flexiblesubstrate, an attachment film, paper including a fibrous material, and abase material film. Examples of a glass substrate include a bariumborosilicate glass substrate, an aluminoborosilicate glass substrate,and a soda lime glass substrate. Examples of a flexible substrate, anattachment film, and a base material film include plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES); a synthetic resin of acrylic or the like; afilm made of polypropylene, polyester, polyvinyl fluoride, polyvinylchloride, or the like; an inorganic film formed by evaporation; a filmmade of polyamide, polyimide, aramid, or epoxy; and paper. Specifically,when a transistor is formed using a semiconductor substrate, a singlecrystal substrate, an SOI substrate, or the like, the transistor canhave few variations in characteristics, size, shape, or the like, highcurrent supply capability, and a small size. A circuit using suchtransistors achieves lower power consumption of the circuit or higherintegration of the circuit.

For the above separation layer, for example, a stacked inorganic filmsof a tungsten film and a silicon oxide film, or an organic resin film ofpolyimide or the like formed over a substrate can be used.

In other words, a transistor may be formed using one substrate, and thentransferred to another substrate. Examples of a substrate to which atransistor is transferred include, in addition to the above-describedsubstrates over which transistors can be formed, a paper substrate, acellophane substrate, an aramid film substrate, a polyimide filmsubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, and a rubber substrate. With the use of such asubstrate, a transistor with excellent properties or low powerconsumption, or a device with high durability can be formed, and thedevice can have high heat resistance and a reduced weight or thickness.

Note that all the circuits which are necessary to realize a desiredfunction can be formed using one substrate (e.g., a glass substrate, aplastic substrate, a single crystal substrate, or an SOI substrate). Inthis manner, the cost can be reduced by a reduction in the number ofcomponents or reliability can be improved by a reduction in the numberof connection points to circuit components.

Note that not all the circuits which are necessary to realize thepredetermined function are needed to be formed using one substrate. Thatis, part of the circuits which are necessary to realize thepredetermined function may be formed using a substrate and another partof the circuits which are necessary to realize the predeterminedfunction may be formed using another substrate. For example, part of thecircuits which are necessary to realize the predetermined function canbe formed using a glass substrate and another part of the circuits whichare necessary to realize the predetermined function can be formed usinga single crystal substrate (or an SOI substrate). The single crystalsubstrate over which the another part of the circuits which arenecessary to realize the predetermined function (such a substrate isalso referred to as an IC chip) can be connected to the glass substrateby COG (chip on glass), and the IC chip can be provided over the glasssubstrate. Alternatively, the IC chip can be connected to the glasssubstrate by TAB (tape automated bonding), COF (chip on film), SMT(surface mount technology), a printed circuit board, or the like. Whenpart of the circuits is thus formed over the same substrate as a pixelportion, the cost can be reduced by a reduction in the number ofcomponents or reliability can be improved by a reduction in the numberof connection points between circuit components. In particular, acircuit in a portion where a driving voltage is high, a circuit in aportion where a driving frequency is high, or the like consumes muchpower in many cases. In view of the above, such a circuit is formed overa substrate (e.g., a single crystal substrate) different from asubstrate over which a pixel portion is formed, whereby an IC chip isformed. The use of this IC chip prevents an increase in powerconsumption.

The transistor 73 can have a bottom-gate structure or a top-gatestructure. Note that examples of the structure of the transistor will bedescribed in detail in the following embodiment.

The conductive layer 75 can be formed using a metal material having alight-reflecting property. For example, the conductive layer 75 can beformed using one or more kinds of materials selected from a metal suchas tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), orsilver (Ag); an alloy of any of these metals; and a nitride of any ofthese metals. The conductive layer 75 may be stacked over a materialhaving a light-transmitting property. The depression and projectionportion 77 may be formed by processing the surface of the conductivelayer 75 by etching or the like, or may be obtained by forming theconductive layer 75 over a structure body such as a conductive layer oran insulating layer.

Note that the structure body under the conductive layer 75 can be formedusing an organic or inorganic material, typically, a visible lightcurable resin, an ultraviolet curable resin, or a thermosetting resin.For example, an acrylic resin, an epoxy resin, or an amine resin can beused. Note that the structure body may have a stacked structure of thinfilms.

For the coloring layer 83, a color filter for transmitting light in ared wavelength range, a color filter for transmitting light in a greenwavelength range, a color filter for transmitting light in a bluewavelength range, or the like can be used.

Each color filter is formed in a desired position with a known materialby a printing method, an inkjet method, an etching method using aphotolithography technique, or the like.

The light-shielding layer 85 is formed in a desired position with aknown material having a light-blocking property by a printing method, aninkjet method, an etching method using a photolithography technique, orthe like

The insulating layer 87 has a function of protecting the coloring layer83 and the light-shielding layer 85. The insulating layer 87 can beformed using an acrylic-based resin or the like.

The conductive layer 89 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

There is no particular limitation on the polarizing plate 91 as long asit can produce linearly polarized light from natural light or circularlypolarized light. For example, a polarizing plate whose opticalanisotropy is obtained by disposing dichroic substances in one directioncan be used. Such a polarizing plate can be formed in such a manner thatan iodine-based compound or the like is adsorbed to a film such as apolyvinyl alcohol film and the film is stretched in one direction. Notethat as the dichroic substance, a dye-based compound or the like as wellas an iodine-based compound can be used.

The light diffusion layer 93 is formed in a desired position with avariety of materials such as a transparent resin to which fine particlesare added.

Although not illustrated in FIGS. 9A and 9B, an optical member such asan alignment film, a half-wave plate, or a quarter-wave plate.

The structures shown in this embodiment can be used in appropriatecombination with any of the structures shown in the other embodiments.Hence, in the liquid crystal display device that is one embodiment ofthe present invention, the power consumed by a backlight can be reduced.In addition, because a reflective liquid crystal display device isobtained, transistors in a pixel can be provided to overlap with a pixelelectrode. Therefore, even when the number of transistors in the pixelincreases, the pixel size is not reduced and display quality can bemaintained.

EMBODIMENT 3

Described in this embodiment are examples of structures of a transistorthat can be used for the liquid crystal display device described above.Here, structure examples of the cross section of the transistor will bedescribed with reference to cross-sectional views of FIGS. 10A to 10C.FIG. 10A is a cross-sectional view of a bottom-gate transistor. FIG. 10Bis a cross-sectional view of a top-gate transistor. FIG. 10C is across-sectional view of a dual-gate transistor.

A transistor 100A illustrated in FIG. 10A includes a conductive layer103, an insulating layer 105, a semiconductor layer 107, a conductivelayer 109A, a conductive layer 109B, an insulating layer 111, and aninsulating layer 113 over a substrate 101.

Note that FIG. 10A shows a cross-sectional view of a channel-etchedtransistor with the bottom-gate structure; however, a channel-protectivetransistor may also be used. The semiconductor layer 107 may have alayered structure although FIG. 10A shows a single-layer semiconductorlayer 107.

A transistor 100B illustrated in FIG. 10B includes the semiconductorlayer 107 provided with a semiconductor layer 107A and a semiconductorlayer 107B, the insulating layer 105, the conductive layer 103, theinsulating layer 111, the insulating layer 113, the conductive layer109A, and the conductive layer 109B over the substrate 101.

Note that FIG. 10B shows, but is not limited to, the top-gate structurein which the conductive layers 109A and 109B are connected to each otherthrough openings formed in the insulating layer 113. For example, theconductive layers 109A and 109B may be formed directly on thesemiconductor layer 107 without provision of the openings.

A transistor 100C illustrated in FIG. 10C includes a conductive layer103A, the insulating layer 105, the semiconductor layer 107, theconductive layer 109A, the conductive layer 109B, the insulating layer111, a conductive layer 103B, and the insulating layer 113 over thesubstrate 101.

The components illustrated in FIGS. 10A to 10C will be described below.

The substrate 101 is similar to the element substrate 71 or the countersubstrate 81 described in the above embodiment, and therefore will notbe described.

For the conductive layers 103, 103A, 103B, 109A, and 109B, a metalelement selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten, an alloy containing any of these metalelements as a component, an alloy containing these metal elements incombination, or the like can be used. Furthermore, one or more metalelements selected from manganese and zirconium may be used. In addition,the conductive layers 103, 103A, 103B, 109A, and 109B may have asingle-layer structure or a stacked-layer structure of two or morelayers. Examples of the structure include a single-layer structure of analuminum film containing silicon, a two-layer structure in which atitanium film is stacked over an aluminum film, a two-layer structure inwhich a titanium film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a titaniumnitride film, a two-layer structure in which a tungsten film is stackedover a tantalum nitride film or a tungsten nitride film, and athree-layer structure in which a titanium film, an aluminum film, and atitanium film are stacked in this order. Alternatively, an alloy film ora nitride film in which aluminum is combined with one or more elementsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

The conductive layers 103, 103A, 103B, 109A, and 109B can be formedusing a light-transmitting conductive material such as indium tin oxide,indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added. It is also possible to use a layeredstructure of the light-transmitting conductive material and theaforementioned metal element.

The insulating layers 105 and 111 each may be formed to have asingle-layer structure or a stacked-layer structure using, for example,one or more of silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, hafnium oxide, gallium oxide,Ga—Zn-based metal oxide, silicon nitride, and the like. The insulatinglayers 105 and 111 each may be formed using a high-k material such ashafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The insulating layer 113 may be formed by a CVD method, a sputteringmethod, or the like to have a single-layer structure or a stacked-layerstructure using, for example, silicon oxide, silicon oxynitride, orsilicon nitride. The insulating layer 113 can also be made using anorganic resin such as an acrylic resin, a polyimide resin, abenzocyclobutene-based resin, a siloxane-based resin, a polyamide resin,or an epoxy resin.

The semiconductor layers 107, 107A, and 107B each may be formed to havea single-layer structure or a stacked-layer structure using, typically,In—Ga oxide, In—Zn oxide, and In-M-Zn oxide (M represents Al, Ti, Ga, Y,Zr, La, Ce, Nd, Sn, or Hf).

The semiconductor layers 107, 107A, and 107B each may be an In-M-Znoxide layer having any of the following atomic ratios of In to M, whenZn and O are not taken into consideration: the atomic percentage of Inis smaller than 50 atomic % and the atomic percentage of M is largerthan or equal to 50 atomic %, and the atomic percentage of In is smallerthan 25 atomic % and the atomic percentage of M is larger than or equalto 75 atomic %.

The energy gap of each of the semiconductor layers 107, 107A, and 107Bis 2 eV or more, preferably 2.5 eV or more, and more preferably 3 eV ormore. The off-state current of the transistor can be reduced by using anoxide semiconductor having such a wide energy gap.

For the semiconductor layers 107, 107A, and 107B, an In—Ga—Zn oxidecontaining In, Ga, and Zn at an atomic ratio of 1:1:1, 1:1:1.2, or 3:1:2can be used. Note that the atomic ratio of each of the semiconductorlayers 107, 107A, and 107B varies within a range of ±20% as an error.

Note that the conductivity of the semiconductor layer 107A is preferablyhigher than that of the semiconductor layer 107B. To increase theconductivity of the semiconductor layer 107A, a rare gas or the likecontaining argon is preferably added to the semiconductor layer 107A ina self-aligned manner with the conductive layer 103 serving as a gateelectrode used as a mask. Such a structure reduces the number of masks.

The structures shown in this embodiment can be used in appropriatecombination with any of the structures shown in the other embodiments.

EMBODIMENT 4

Described in this embodiment will be one embodiment that can be appliedto an oxide semiconductor film in the transistor included in the liquidcrystal display device described in the above embodiments.

The oxide semiconductor film may include one or more of the following:an oxide semiconductor having a single-crystal structure (hereinafterreferred to as a single-crystal oxide semiconductor); an oxidesemiconductor having a polycrystalline structure (hereinafter referredto as a polycrystalline oxide semiconductor); an oxide semiconductorhaving a microcrystalline structure (hereinafter referred to as amicrocrystalline oxide semiconductor), and an oxide semiconductor havingan amorphous structure (hereinafter referred to as an amorphous oxidesemiconductor). Further, the oxide semiconductor film may be formedusing a CAAC-OS film. Furthermore, the oxide semiconductor film mayinclude an amorphous oxide semiconductor and an oxide semiconductorhaving a crystal grain. A CAAC-OS film and a microcrystalline oxidesemiconductor film are described below as typical examples.

First, a CAAC-OS film is described.

The CAAC-OS film is an oxide semiconductor film having a plurality ofc-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachlayer of metal atoms has a morphology reflecting a surface over whichthe CAAC-OS film is formed (hereinafter, a surface over which theCAAC-OS film is formed is referred to as a formation surface) or a topsurface of the CAAC-OS film, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan-view TEM image), metal atoms are arranged in a triangularor hexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

FIG. 15 a is a cross-sectional TEM image of a CAAC-OS film. FIG. 15 b isa cross-sectional TEM image obtained by enlarging the image of FIG. 15a. In FIG. 15 b, atomic arrangement is highlighted for easyunderstanding.

FIG. 15 c is local Fourier transform images of regions each surroundedby a circle (the diameter is about 4 nm) between A and O and between Oand A′ in FIG. 15 a. C-axis alignment can be observed in each region inFIG. 15 c. The c-axis direction between A and O is different from thatbetween O and A′, which indicates that a grain in the region between Aand O is different from that between O and A′. In addition, the angle ofthe c-axis between A and O continuously and gradually changes, forexample, 14.3°, 16.6°, and 26.4°. Similarly, the angle of the c-axisbetween O and A′ continuously changes, for example, −18.3°, −17.6°, and−15.9°.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) indicating alignment are shown. For example, whenelectron diffraction with an electron beam having a diameter of 1 nm ormore and 30 nm or less (such electron diffraction is also referred to asnanobeam electron diffraction) is performed on the top surface of theCAAC-OS film, spots are observed (see FIG. 16A).

From the results of the cross-sectional TEM image and the plan-view TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in theplan-view TEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears frequentlywhen 2θ is around 56°. This peak is derived from the (110) plane of theInGaZnO₄ crystal. Here, analysis (φ scan) is performed under conditionswhere the sample is rotated around a normal vector of a sample surfaceas an axis (φ axis) with 2θ fixed at around 56°. In the case where thesample is a single crystal oxide semiconductor film of InGaZnO₄, sixpeaks appear. The six peaks are derived from crystal planes equivalentto the (110) plane. On the other hand, in the case of a CAAC-OS film, apeak is not clearly observed even when φ scan is performed with 2θ fixedat around 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment.

As described above, the c-axis of the crystal is aligned in a directionparallel to a normal vector of a formation surface or a normal vector ofa top surface. Thus, for example, in the case where the shape of theCAAC-OS film is changed by etching or the like, the c-axis might not benecessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Distribution of c-axis aligned crystal parts in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases.Furthermore, when an impurity is added to the CAAC-OS film, a region towhich the impurity is added may be altered and the proportion of thec-axis aligned crystal parts in the CAAC-OS film might vary depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak may also be observed when 2θ is around36°, in addition to the peak at 2θ of around 31°. The peak at 2θ ofaround 36° indicates that a crystal having no c-axis alignment isincluded in part of the CAAC-OS film. It is preferable that in theCAAC-OS film, a peak appear when 2θ is around 31° and that a peak notappear when 2θ is around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Furthermore, a heavymetal such as iron or nickel, argon, carbon dioxide, or the like has alarge atomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Therefore, atransistor including the oxide semiconductor film rarely has negativethreshold voltage (is rarely normally on). The highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasfew carrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released and mightbehave like fixed electric charge. Thus, the transistor including theoxide semiconductor film having high impurity concentration and a highdensity of defect states has unstable electrical characteristics in somecases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film is described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor film in some cases. In mostcases, the size of a crystal part in the microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. A microcrystal with a size greater than or equal to 1 nm andless than or equal to 10 nm, or a size greater than or equal to 1 nm andless than or equal to 3 nm is specifically referred to as nanocrystal(nc). An oxide semiconductor film including nanocrystal is referred toas a nanocrystalline oxide semiconductor (nc-OS) film. In an imageobtained with TEM, a grain boundary cannot be found clearly in the nc-OSfilm in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak showing a crystalplane does not appear. A diffraction pattern like a halo pattern appearsin a selected-area electron diffraction pattern of the nc-OS filmobtained by using an electron beam having a probe diameter (e.g., largerthan or equal to 50 nm) larger than the diameter of a crystal part.Meanwhile, spots are shown in a nanobeam electron diffraction pattern ofthe nc-OS film obtained by using an electron beam having a probediameter close to, or smaller than the diameter of a crystal part. In ananobeam electron diffraction pattern of the nc-OS film, regions withhigh luminance in a circular (ring) pattern are shown in some cases.Furthermore, in a nanobeam electron diffraction pattern of the nc-OSfilm, a plurality of circumferentially distributed spots are observed insome cases (see FIG. 16B).

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Thus, the nc-OSfilm has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film. Hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more kinds of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

In the case where an oxide semiconductor film has a plurality ofstructures, the structures can be analyzed using nanobeam electrondiffraction in some cases.

FIG. 16C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 170, an optical system172 below the electron gun chamber 170, a sample chamber 174 below theoptical system 172, an optical system 176 below the sample chamber 174,an observation chamber 180 below the optical system 176, a camera 178installed in the observation chamber 180, and a film chamber 182 belowthe observation chamber 180. The camera 178 is provided to face towardthe inside of the observation chamber 180. Note that the film chamber182 is not necessarily provided.

FIG. 16D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 16C. In thetransmission electron diffraction measurement apparatus, a substance 188which is positioned in the sample chamber 174 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 170 through the optical system 172. Electrons passing throughthe substance 188 are incident on a fluorescent plate 192 provided inthe observation chamber 180 through the optical system 176. On thefluorescent plate 192, a pattern corresponding to the intensity of theincident electrons appears, which allows measurement of a transmissionelectron diffraction pattern.

The camera 178 is installed so as to face the fluorescent plate 192 andcan take an image of a pattern appearing on the fluorescent plate 192.An angle formed by a straight line which passes through the center of alens of the camera 178 and the center of the fluorescent plate 192 andan upper surface of the fluorescent plate 192 is, for example, 15° ormore and 80° or less, 30° or more and 75° or less, or 45° or more and70° or less. As the angle is reduced, distortion of the transmissionelectron diffraction pattern taken by the camera 178 becomes larger.Note that if the angle is obtained in advance, the distortion of anobtained transmission electron diffraction pattern can be corrected.Note that the film chamber 182 may be provided with the camera 178. Forexample, the camera 178 may be set in the film chamber 182 so as to beopposite to the incident direction of electrons 184. In this case, atransmission electron diffraction pattern with less distortion can betaken from the rear surface of the fluorescent plate 192.

A holder for fixing the substance 188 that is a sample is provided inthe sample chamber 174. The holder transmits electrons passing throughthe substance 188. The holder may have, for example, a function ofmoving the substance 188 in the direction of the X, Y, and Z axes. Themovement function of the holder may have an accuracy of moving thesubstance in the range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10nm to 100 nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range ispreferably determined to be an optimal range for the structure of thesubstance 188.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above will be described.

For example, changes in the structure of a substance can be observed bychanging the irradiation position of the electrons 184 that are ananobeam in the substance (or by scanning) as illustrated in FIG. 16D.At this time, when the substance 188 is a CAAC-OS film, a diffractionpattern shown in FIG. 16A is observed. When the substance 188 is annc-OS film, a diffraction pattern shown in FIG. 16B is observed.

Even when the substance 188 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether a CAAC-OS film is favorable can be determinedby the proportion of a region where a diffraction pattern of a CAAC-OSfilm is observed in a predetermined area (also referred to as proportionof CAAC). In the case of a high quality CAAC-OS film, for example, theproportion of CAAC is higher than or equal to 50%, preferably higherthan or equal to 80%, more preferably higher than or equal to 90%, andstill preferably higher than or equal to 95%. Note that the proportionof a region where a diffraction pattern different from that of a CAAC-OSfilm is observed is referred to as the proportion of non-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedimmediately after deposition (represented as “as-sputtered”) and a topsurface of a sample including a CAAC-OS subjected to heat treatment at450° C. in an atmosphere containing oxygen. Here, the proportion of CAACwas obtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/s and the obtained diffractionpatterns were converted into still images every 0.5 seconds. Note thatas an electron beam, a nanobeam with a probe diameter of 1 nm was used.The above measurement was also performed on six samples. The proportionof CAAC was calculated using the average value of the six samples.

FIG. 17A shows the proportion of CAAC in each sample. The proportion of

CAAC in the CAAC-OS film obtained immediately after the deposition was75.7% (the proportion of non-CAAC was 24.3%). The proportion of CAAC inthe CAAC-OS film subjected to the heat treatment at 450° C. was 85.3%(the proportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained immediately after the deposition. That is,heat treatment at high temperature (e.g., higher than or equal to 400°C.) reduces the proportion of non-CAAC (increases the proportion ofCAAC). The above results also indicate that even when the temperature ofthe heat treatment is lower than 500° C., the CAAC-OS film can have ahigh proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmwere similar to that of an nc-OS film. Furthermore, an amorphous oxidesemiconductor film was not able to be observed in a measurement region.Thus, the results suggest that a region having a structure similar tothat of an nc-OS film is rearranged by the heat treatment owing to theinfluence of the structure of an adjacent region, so that the regionbecomes CAAC.

FIGS. 17B and 17C are planar TEM images of the CAAC-OS film obtainedimmediately after the deposition and the CAAC-OS film subjected to theheat treatment at 450° C., respectively. Comparison between FIGS. 17Band 17C shows that the

CAAC-OS film subjected to the heat treatment at 450° C. has more evenfilm quality. That is, the heat treatment at high temperature improvesthe film quality of the CAAC-OS film.

With such a measurement method, the structure of an oxide semiconductorfilm having a plurality of structures can be analyzed in some cases.

The structures, methods, and the like shown in this embodiment can beused in appropriate combination with any of the structures, methods, andthe like shown in the other embodiments.

EMBODIMENT 5

As described in Embodiment 1, with the pixel structure of the liquidcrystal display device that is disclosed in one embodiment of thepresent invention, a written data voltage can be held during a longperiod and the writing interval can be increased. The liquid crystaldisplay device of this embodiment can be driven by at least two methods(modes) to display images. The first driving mode is a conventionaldriving method of a liquid crystal display device, in which data isrewritten every frame. In the second driving mode, data rewriting isstopped after data writing is executed, that is, the refresh rate isreduced.

Moving images are displayed in the first driving mode. Still images aredisplayed in the second driving mode. In the second driving mode, drivercircuits do not need to be operated for driving pixels, reducing powerconsumption.

In addition, a pixel electrode of the liquid crystal display device ofone embodiment of the present invention has a function of reflectingexternal light. In other words, the liquid crystal display device of oneembodiment of the present invention is a so-called reflective liquidcrystal display device. Hence, displayed images can be made visiblewithout a need of a backlight, resulting in lower power consumption.

An effect of reducing the refresh rate will be described here.

The eye strain is divided into two categories: nerve strain and musclestrain. The nerve strain is caused by prolonged looking at light emittedfrom a liquid crystal display device or blinking images. This is becausethe brightness stimulates and fatigues the retina and nerve of the eyeand the brain. The muscle strain is caused by overuse of a ciliarymuscle which works for adjusting the focus.

FIG. 11A is a schematic diagram illustrating display of a conventionalliquid crystal display device. As illustrated in FIG. 11A, for thedisplay of the conventional liquid crystal display device, imagerewriting is performed 60 times per second. A prolonged looking at sucha screen might stimulate the retina and nerve of the eye and the brainof a user and lead to eye strain.

In contrast, when images are displayed in the liquid crystal displaydevice of one embodiment of the present invention, for example, thenumber of times of image writing can be reduced to once every fiveseconds as shown in FIG. 11B. In that case, unlike in FIG. 11A, the sameimage can be seen for as long as possible and flickers on a screenperceived by a user can be reduced. This makes it possible to reducestimuli to the retina and nerve of the eye and the brain of a user,resulting in less nerve strain.

One embodiment of the present invention can provide an eye-friendlyliquid crystal display device.

The structures, methods, and the like shown in this embodiment can beused in appropriate combination with any of the structures, methods, andthe like shown in the other embodiments.

EMBODIMENT 6

In this embodiment, structure examples of electronic devices eachincluding a liquid crystal display device of one embodiment of thepresent invention will be described. Furthermore, in this embodiment, adisplay module including a liquid crystal display device of oneembodiment of the present invention will be described with reference toFIG. 12.

In a display module 8000 illustrated in FIG. 12, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.Note that the backlight unit 8007, the battery 8011, the touch panel8004, and the like are not provided in some cases.

The liquid crystal display device of one embodiment of the presentinvention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel. An electrode fora touch sensor may be provided in each pixel of the display panel 8006so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 13A to 13H and FIGS. 14A to 14D illustrate electronic devices.These electronic devices can include a housing 5000, a display portion5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including apower switch or an operation switch), a connection terminal 5006, asensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, smell, or infrared ray), a microphone 5008, and the like.

FIG. 13A illustrates a portable computer, which can include a switch5009, an infrared port 5010, and the like in addition to the aboveobjects. FIG. 13B illustrates a portable image reproducing deviceprovided with a memory medium (e.g., a DVD reproducing device), whichcan include a second display portion 5002, a memory medium read portion5011, and the like in addition to the above objects. FIG. 13Cillustrates a goggle-type display, which can include the second displayportion 5002, a support 5012, an earphone 5013, and the like in additionto the above objects. FIG. 13D illustrates a portable game machine,which can include the memory medium read portion 5011 and the like inaddition to the above objects. FIG. 13E illustrates a digital camerawith a television reception function, which can include an antenna 5014,a shutter button 5015, an image reception portion 5016, and the like inaddition to the above objects. FIG. 13F illustrates a portable gamemachine, which can include the second display portion 5002, the memorymedium read portion 5011, and the like in addition to the above objects.FIG. 13G illustrates a television receiver, which can include a tuner,an image processing portion, and the like in addition to the aboveobjects. FIG. 13H illustrates a portable television receiver, which caninclude a charger 5017 capable of transmitting and receiving signals,and the like in addition to the above objects. FIG. 14A illustrates adisplay, which can include a support base 5018 and the like in additionto the above objects. FIG. 14B illustrates a camera, which can includean external connection port 5019, a shutter button 5015, an imagereception portion 5016, and the like in addition to the above objects.FIG. 14C illustrates a computer, which can include a pointing device5020, the external connection port 5019, a reader/writer 5021, and thelike in addition to the above objects. FIG. 14D illustrates a mobilephone, which can include a transmitter, a receiver, a tuner of 1 segpartial reception service for mobile phones and mobile terminals, andthe like in addition to the above objects.

The electronic devices illustrated in FIGS. 13A to 13H and FIGS. 14A to14D can have a variety of functions, for example, a function ofdisplaying a variety of data (a still image, a moving image, a textimage, and the like) on the display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling a process with a variety of software (programs), awireless communication function, a function of being connected to avariety of computer networks with a wireless communication function, afunction of transmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Furthermore, the electronic device including aplurality of display portions can have a function of displaying imagedata mainly on one display portion while displaying text data on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images on a plurality of display portions with a parallaxtaken into account, or the like. Furthermore, the electronic deviceincluding an image receiving portion can have a function of shooting astill image, a function of taking a moving image, a function ofautomatically or manually correcting a shot image, a function of storinga shot image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying a shotimage on the display portion, or the like. Note that functions that canbe provided for the electronic devices illustrated in FIGS. 13A to 13Hand FIGS. 14A to 14D are not limited to those described above, and theelectronic devices can have a variety of functions.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data.

Next, application examples of the liquid crystal display device will bedescribed.

FIG. 14E illustrates an example in which a liquid crystal display deviceis incorporated in a building structure. FIG. 14E illustrates a housing5022, a display portion 5023, a remote control 5024 that is an operationportion, a speaker 5025, and the like. The liquid crystal display deviceis incorporated in the building structure as a wall-hanging type and canbe provided without requiring a large space.

FIG. 14F illustrates another example in which a liquid crystal displaydevice is incorporated in a building structure. A display module 5026 isincorporated in a prefabricated bath unit 5027, so that a bather canview the display module 5026.

Note that this embodiment shows, but is not limited to, the wall and theprefabricated bath unit as examples of the building structures. Theliquid crystal display devices can be provided in a variety of buildingstructures.

Next, examples in which liquid crystal display devices are incorporatedin moving objects are described.

FIG. 14G shows an example in which a liquid crystal display device isincorporated in a car. A display module 5028 is incorporated in a carbody 5029 of the car and can display information related to theoperation of the car or information input from the inside or outside ofthe car on demand. Note that the display module 5028 may have anavigation function.

FIG. 14H shows an example in which a liquid crystal display device isincorporated in a passenger airplane. FIG. 14H illustrates a usagepattern when a display module 5031 is provided for a ceiling 5030 abovea seat of the passenger airplane. The display module 5031 isincorporated in the ceiling 5030 through a hinge portion 5032, and apassenger can view the display module 5031 by stretching of the hingeportion 5032. The display module 5031 has a function of displayinginformation by the operation of the passenger.

Note that this embodiment shows, but is not limited to, bodies of a carand an airplane as examples of moving objects. The liquid crystaldisplay devices can be provided for a variety of objects such astwo-wheeled vehicles, four-wheeled vehicles (including cars, buses, andthe like), trains (including monorails, railroads, and the like), andvessels.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, it is possible to take out part of thediagram or the text and constitute an embodiment of the invention. Thus,in the case where a diagram or a text related to a certain portion isdescribed, the context taken out from part of the diagram or the text isalso disclosed as one embodiment of the invention, and one embodiment ofthe invention can be constituted. Therefore, for example, in a diagramor a text in which one or more active elements (e.g., transistors ordiodes), wirings, passive elements (e.g., capacitors or resistors),conductive layers, insulating layers, semiconductor layers, organicmaterials, inorganic materials, components, devices, operating methods,manufacturing methods, or the like are described, it is possible to takeout part of the diagram or the text and constitute one embodiment of theinvention. For example, from a circuit diagram in which N circuitelements (e.g., transistors or capacitors; N is an integer) areprovided, it is possible to constitute one embodiment of the inventionby taking out M circuit elements (e.g., transistors or capacitors; M isan integer, where M<N). As another example, it is possible to constituteone embodiment of the invention by taking out M layers (M is an integer,where M<N) from a cross-sectional view in which N layers (N is aninteger) are provided. As another example, it is possible to constituteone embodiment of the invention by taking out M elements (M is aninteger, where M<N) from a flow chart in which N elements (N is aninteger) are provided.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, in the case where at least one specificexample is described, it will be readily appreciated by those skilled inthe art that a broader concept of the specific example can be derived.Thus, in the diagram or the text described in one embodiment, in thecase where at least one specific example is described, a broader conceptof the specific example is disclosed as one embodiment of the invention,and one embodiment of the invention can be constituted.

Note that in this specification and the like, content described in atleast a diagram (or may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Thus, when certain content is described in a diagram, thecontent is disclosed as one embodiment of the invention even when thecontent is not described with a text, and one embodiment of theinvention can be constituted. Similarly, part of a diagram that is takenout from the diagram is disclosed as one embodiment of the invention,and one embodiment of the invention can be constituted.

This application is based on Japanese Patent Application serial No.2013-270825 filed with Japan Patent Office on Dec. 27, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising a pixel comprising: asignal line configured to be supplied with a data voltage having a firstpotential and a second potential; a first circuit capable of holding thefirst potential at a first node and electrically connected to the signalline; a second circuit capable of holding the second potential at asecond node and electrically connected to the first circuit; a firsttransistor of which a gate terminal is electrically connected to thefirst node and configured to be supplied with the first potential; asecond transistor of which a gate terminal is electrically connected tothe second node and configured to be supplied with the second potential;a display element of which one electrode is electrically connected to afirst terminal of the first transistor and a first terminal of thesecond transistor; a first wiring configured to be supplied with a firstsignal and electrically connected to a second terminal of the firsttransistor; and a second wiring configured to be supplied with a secondsignal and electrically connected to a second terminal of the secondtransistor.
 2. The display device according to claim 1, wherein thefirst circuit comprises: a third transistor of which a gate terminal iselectrically connected to a scan signal line, a first terminal iselectrically connected to the signal line, and a second terminal iselectrically connected to the first node, and wherein the second circuitcomprises: a fourth transistor of which a gate terminal is electricallyconnected to the first node and a first terminal is electricallyconnected to the second node; and a fifth transistor of which a gateterminal is electrically connected to a reset signal line and a firstterminal is electrically connected to the second node.
 3. The displaydevice according to claim 2, wherein each of the third to fifthtransistors comprises a channel formation region in an oxidesemiconductor layer.
 4. The display device according to claim 3, whereinan off-state current of each of the third to fifth transistors is lessthan or equal to 10 zA per micrometer of a channel width.
 5. The displaydevice according to claim 2, wherein the first circuit comprises a firstcapacitor of which one electrode is electrically connected to the firstnode, and wherein the second circuit comprises a second capacitor ofwhich one electrode is electrically connected to the second node.
 6. Thedisplay device according to claim 5, wherein the other electrode of thefirst capacitor and the other electrode of the second capacitor areelectrically connected to a third wiring configured to be supplied withone of the first potential and the second potential.
 7. The displaydevice according to claim 1, wherein the first potential is higher thanthe second potential.
 8. The display device according to claim 1,wherein a potential of the first signal is lower than the firstpotential and higher than the second potential.
 9. The display deviceaccording to claim 1, wherein the display element comprises a liquidcrystal layer, and wherein the one electrode of the display element is areflective electrode.
 10. The display device according to claim 9,wherein the first signal allows light to pass through the liquid crystallayer, and wherein the second signal prevents light from passing throughthe liquid crystal layer.
 11. The display device according to claim 1,wherein the display element comprises a light-emitting layer.
 12. Anelectronic device comprising the display device according to claim 1.13. A liquid crystal display device comprising a pixel comprising: asignal line configured to be supplied with a data voltage having a firstpotential and a second potential; a reset signal line configured to besupplied with a reset signal; a scan signal line configured to besupplied with a scan signal; a first circuit comprising a first node andelectrically connected to the signal line and the scan signal line; asecond circuit comprising a second node and electrically connected tothe reset signal line; a first transistor of which a gate terminal iselectrically connected to the first node; a second transistor of which agate terminal is electrically connected to the second node; and a liquidcrystal element of which one electrode is electrically connected to afirst terminal of the first transistor and a first terminal of thesecond transistor, wherein the first circuit is configured so that thesecond potential is held at the first node in response to supply of thescan signal to the first circuit and then one of the first potential andthe second potential is held at the first node in response to supply ofthe data voltage to the first circuit, wherein the second circuit isconfigured so that the first potential is held at the second node inresponse to supply of the reset signal to the second circuit and thenthe other of the first potential and the second potential is held at thesecond node in response to the supply of the data voltage to the firstcircuit, and wherein the first potential is higher than the secondpotential.
 14. The liquid crystal display device according to claim 13,wherein a second terminal of the first transistor is electricallyconnected to a first wiring configured to be supplied with a firstsignal which allows light to pass through a liquid crystal layer,wherein a second terminal of the second transistor is electricallyconnected to a second wiring configured to be supplied with a secondsignal which prevents light from passing through the liquid crystallayer, and wherein the one electrode of the liquid crystal element is areflective electrode.
 15. The liquid crystal display device according toclaim 14, wherein a potential of the first signal is lower than thefirst potential and higher than the second potential.
 16. The liquidcrystal display device according to claim 13, wherein the first circuitcomprises: a third transistor of which a gate terminal is electricallyconnected to the scan signal line, a first terminal is electricallyconnected to the signal line, and a second terminal is electricallyconnected to the first node, and wherein the second circuit comprises: afourth transistor of which a gate terminal is electrically connected tothe first node and a first terminal is electrically connected to thesecond node; and a fifth transistor of which a gate terminal iselectrically connected to the reset signal line and a first terminal iselectrically connected to the second node.
 17. The liquid crystaldisplay device according to claim 16, wherein each of the third to fifthtransistors comprises a channel formation region in an oxidesemiconductor layer.
 18. The liquid crystal display device according toclaim 17, wherein an off-state current of each of the third to fifthtransistors is less than or equal to 10 zA per micrometer of a channelwidth.
 19. The liquid crystal display device according to claim 16,wherein the first circuit comprises a first capacitor of which oneelectrode is electrically connected to the first node, and wherein thesecond circuit comprises a second capacitor of which one electrode iselectrically connected to the second node.
 20. An electronic devicecomprising the liquid crystal display device according to claim 13.